The present invention is related to a vertical-cavity, surface-emission-type laser diode and the process of making the same. Further, the present invention relates to a vertical-cavity, surface-emission-type laser-diode array, an optical transmission module, an optical transceiver module and also an optical telecommunication system.
Vertical-cavity, surface-emission-type laser diode is a laser diode that emits an optical beam in a vertical direction to a substrate. It is used for a light source of optical interconnection systems and optical pickup devices, and the like.
A vertical-cavity, surface-emission-type laser diode has an active region including an active layer that produces a laser beam. The active region is sandwiched with a pair of reflectors, wherein a semiconductor distributed Bragg reflector, in which a low-refractive index layer and a high refractive index layer are laminated alternately, is used widely for the reflectors. Materials having a wider bandgap than the active layer and not causing absorption of the optical beam coming from an active layer are used for the semiconductor distributed Bragg reflector. Particularly, the materials that achieve a lattice matching with the substrate are used so as to avoid lattice relaxation.
Meanwhile, the reflector has to have a high reflectance of 99% or more. Generally, the reflectance of the reflector becomes higher by increasing the number of stacking. However, production of the vertical-cavity, surface-emission-type laser diode becomes difficult when the number of stacking in the reflectors is increased excessively. Because of this, it is preferable that there exists a large refractive index difference between the low-refractive index layer and the high refractive index layer constituting the semiconductor distributed Bragg reflectors. AlAs and GaAs are end-member compositions of the system AlGaAs having a lattice constant almost the same as that of GaAs. Further, the materials of this system can provide a large refractive index difference therebetween. Because of this reason, it is possible to achieve a high reflectance with fewer number of stacking by using the material of the AlGaAs system. Thus, the material of the AlGaAs system is used widely.
However, the material containing Al is very reactive, and crystal defects, originating from Al, are formed easily. For example, oxygen molecules or water molecules contained in the source material or growth atmosphere are easily incorporated into the crystal as a result of reaction with Al. Once they are thus incorporated, they form a crystal defect acting as non-optical recombination center, resulting in degradation of efficacy of optical emission. Further, there is a concern that the reliability of the device may be degraded due to the existence of such crystal defects.
Even when the active region is formed by a layer not containing Al, the problem of non-optical recombination still occurs when Al is contained in the low-refractive index layer (formed of a widegap layer) of the reflector that makes a contact with the active region. More specifically, such a non-optical recombination may occur at the interface between the active region and the reflector when carriers are injected for recombination. Thereby, the efficacy of optical emission falls off inevitably. In order to avoid this adversary influence, it is necessary to carry out rigorous process control, material purity control, optimization of growth condition, and the like. Still, it is not easy to produce a device with high quality.
Meanwhile, there are proposals in Japanese Laid-Open Patent Applications 08-340146 and 07-307525 to form a semiconductor distributed Bragg reflector by using GaInP and GaAs, which are free from Al. However, the difference of refractive index between GaInP and GaAs is only one-half the refractive index difference between AlAs and GaAs. Because of this, the number of stacking in the reflector has to be increased significantly, and the production of the laser diode becomes difficult. Associated with this there arise various problems such as degradation of yield, increased device resistance, increased time needed for producing a laser diode. Further, because of the increase of total thickness, there appears a difficulty in providing electric interconnection in such a laser diode.
Meanwhile, it is practiced to use a current confinement structure in the art of laser diode for reducing the threshold of laser oscillation. Japanese Laid-Open Patent Application 7-240506 discloses a structure that uses a current confinement structure including a high resistance layer formed by an ion implantation process in combination with a semiconductor distributed Bragg reflector that consists of AlAs/GaAs. Further, Japanese Patent 2,917,971 proposes a vertical-cavity, surface-emission-type laser diode that uses, in addition to an optical cavity formed by the semiconductor distributed Bragg reflectors of the AlGaAs/GaAs stacked structure, a current confinement structure that includes an oxide film formed by selective oxidization of a part of the Al(Ga)As optical cavity structure. In this proposal, the oxidation is conducted by supplying water vapor of high temperature. It should be noted that the oxidation process using water vapor of high temperature is capable of forming a true insulator of AlxOy. According to such an approach, the distance between the active layer and the current confinement layer can controlled exactly by controlling the process of crystal growth. Further, it is possible narrow the current path significantly. In view of these, the foregoing construction is suited for reducing reactive current and for reducing the active region. Because of this, it is also suited to for reducing electric power consumption. Thus, the construction is used widely recently.
It should be noted that the foregoing Japanese Patent 2,917,971 uses the phenomenon that the oxidation rate starts to increase sharply as the Al content in the AlGaAs layer is increased. Thus, in order to ensure that only the part to be oxidized is oxidized, the foregoing process increases the Al content of the layer in which the oxidation is to be caused. In this way, it is possible to obtain a current confinement structure by a selective oxidation process. In view of this, the Al content of the AlGaAs layer forming the low-refractive index layer of the semiconductor distributed Bragg reflector is set smaller (Ga content is increased) than the Al content of the Al(Ga)As/GaAs oxidation layer. The composition of AlxGa1-xAs (x=0.97) is used for the selectively oxidized layer in the foregoing Japanese Patent 2,917,971, while a composition of AlxGa1-xAs (x=0.92) is used for the low-refractive index layer of the semiconductor distributed Bragg reflector.
In the art of forming a current confinement structure by such a selective oxidation process, an approach is adopted to oxidize an AlAs layer from a sidewall surface thereof. In order that such a process is to be conducted, it is necessary to remove unnecessary layers by means of a mesa etching process such that the sidewall surface of the AlAs layer to be oxidized is exposed. However, in view of variation in the etching rate, there may be caused variation of mesa height within a lot. Further, there may be caused a lot-to-lot variation of mesa height. When such a variation has been caused, the device characteristic may be scattered correspondingly.
Current optical-fiber telecommunication technology uses a laser diode of long wavelength band of 1.3 μm or 1.55 μm for utilizing the wavelength slot of quartz optical fibers in which the optical loss is minimum. The optical fiber telecommunication system is spreading rapidly and it is expected that it may reach a subscriber terminal (Fiber To The Home; FTTH) in a near future. Furthermore, the technology of information transmission by way of optical signals is going to be introduced even to a device-to-device interconnection system inside an apparatus or even to an interconnection system inside a device. Like this, the technology of information transmission will become important still more. In order to realize such an optical interconnection system, it is essential to realize an optical telecommunication module of unprecedented low-cost. Thus, there is a keen demand for a small, long wavelength-band laser diode of low electric power consumption, with excellent temperature characteristics, capable of eliminating the need of a cooling system.
Currently, the material of GaInPAs system formed on an InP substrate, which is a group III-V semiconductor material, monopolizes the market. It should be noted that the material of the GaInPAs system can be tuned to the foregoing wavelength band. However, the material of the InP system has a drawback, because of the small discontinuity in the conduction band between the cladding layer (spacer layer) and the active layer, in that the electrons injected into the active layer are poorly confined, particularly when temperature of the device is increased. This results in a decrease of efficiency. Further, the materials that achieve lattice matching with an InP substrate cannot provide large refractive index difference suitable for realizing a semiconductor distributed Bragg reflector. As a result, the vertical-cavity, surface-emission-type laser diode of the long wavelength having a performance suitable for practical use has not been obtained.
The material of the GaInNAs system formed on a GaAs substrate is proposed in the Japanese Laid-Open Patent Application No. 6-37355, as the material that can settle the foregoing problems. It should be noted that GaInNAs is a group III-V mixed crystal containing N in addition to other group V element. In the system of GaInNAs, it is possible to achieve lattice matching with a GaAs substrate by adding N to GaInAs having a lattice constant larger than that of GaAs. By doing so, the bandgap energy is reduced also. Thus, it becomes possible to realize optical emission at the wavelength band of 1.3 μm or 1.5 μm. Kondou, et al., calculated the band lineup of this system in the article, Jpn. J. Appl. Phys. Vol. 35 (1996), pp. 1273-1275. As this is a material that can achieve lattice matching with GaAs, a large band discontinuity can be realized by using AlGaAs for the cladding layer. Because of this, there is an expectation that a laser diode having a high characteristic temperature may be realized by using such a material. Further, it should be noted that the material of GaInNAs can be formed on a GaAs substrate. Thus, it becomes possible to construct a/the semiconductor multilayer reflector by using an Al(Ga)As/GaAs material system. Thereby, it becomes possible to reduce the number of stacking in the multilayer reflector significantly as compared with the case of forming the multilayer reflector on the InP substrate. Further, it becomes possible to form an AlAs selective-oxidation layer as the current confinement structure, and the operating current is reduced effectively.
However, the problem noted above arises in the case the material system of Al(Ga)As/GaAs is used for the semiconductor multilayer reflector, as proposed in the Japanese Laid-Open Patent Application 10-303515 or Japanese Laid-Open Patent Application 11-145560. Further, the problem similar to above arises also in the case an AlAs selective-oxidation layer is used for the current confinement structure.